Emmanuel Delhaize and Peter R. Ryan
Division of Plant Industry, Commonwealth Scientific and Industrial Research Organization, Australia

The most easily recognized symptom of Al toxicity is the inhibition of root growth, and this has become a widely accepted measure of A1 stress in plants. In simple nutrient solutions micromolar concentrations of Al can begin to inhibit root growth within 60 min. However, the inhibition of growth per se offers little information about the causes of stress that will either precede or coincide with changes in growth. To understand the mechanisms of Al toxicity, it is essential to identify the primary sites involved, both anatomical and metabolic, being mindful that A1 could have diverse effects and act differently in different species.

Several reviews on Al toxicity are available (see Haug, 1984; Taylor, 1988; Rengel, 1992a); here we limit our discussion to the sites of A1 toxicity in higher plants and to the possible role of Ca in the primary mechanism of Al toxicity.

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